1. Field of the Invention
The field of art to which this invention pertains is catalyst regeneration. The present invention will be most useful in a process for regenerating spent FCC catalyst, but should be useful in any process wherein coke is burned from a solid, particulated, fluidizable catalyst.
2. Description of the Prior Art
Much of the world's crude oil is subjected to fluid catalytic cracking (hereafter FCC) processes to convert the heavier material into lighter products. The fluid catalyst used in these processes is quickly contaminated with coke, and to permit the reuse of the catalyst in the process the coke must be burned from the catalyst. Thus there is usually associated with every FCC process unit a fluid catalyst regenerator.
In the past, the catalyst regenerators have burned coke from the catalyst by adding air to a single regeneration zone. Coke was burned to provide a mixture of carbon monoxide and carbon dioxide. Regeneration was usually incomplete, because adding the stoichiometric requirement of air to the catalyst regeneration zone invariably resulted in considerable oxidation of carbon monoxide to carbon dioxide, usually in an upper portion of the regeneration vessel where no catalyst was available to act as a heat sink. This burning of carbon monoxide to carbon dioxide, often called "afterburning", resulted in extremely high temperatures which could damage the regenerator, hence air addition was restricted to protect the apparatus.
Recently, there have been attempts made to promote combustion of CO within the regenerator to recover the heat liberated by this combustion, to permit use of this heat in the FCC process and to permit more thorough regeneration of the catalyst.
Examples of these recent regeneration processes are Stine et al U.S. Pat. No. 3,844,973 and Horecky, Jr. et al U.S. Pat. No. 3,909,392, the teachings of both of which patents are incorporated by reference.
In the catalyst regeneration apparatus of Conner et al, U.S. Pat. No. 3,893,812 (U.S. Class 23/288 B), the teachings of which are incorporated herein by reference, an improved regenerator design is disclosed. A first intermediate density zone or combustor, i.e. a zone containing fluid catalyst of intermediate density, receives spent catalyst and air, permitting most of the coke to be burned therein. Catalyst and regeneration gases, and CO produced during coke combustion, are then passed upwardly through a dilute-phase transport riser wherein significant amounts of the CO are burned to CO.sub.2. Finally the regenerated catalyst is collected in a second dense bed. This process provides for recycle of a portion of the hot regenerated catalyst from the second dense bed to the combustor via an external catalyst recycle means. The function of catalyst recycle is to increase the temperature in the combustor and increase the rate of coke oxidation. It is also known to provide for internal catalyst recirculation from the second dense bed to the combustor.
Another example of a process operating with two dense beds of catalyst, separated by a dilute phase transport riser, is German OS 25 26 839, corresponding to U.S. Ser. No. 479,726, filed June 17, 1974 (Class 252), the teachings of which are incorporated by reference. In this reference hot regenerated catalyst from the second dense bed is admixed with spent catalyst from the FCC reactor in a riser beneath the first dense bed or combustor of the FCC regenerator. Dilute-phase conditions are maintained in the riser (item 34 in the drawing of the U.S. application) by the addition of sufficient air. The dilute-phase condition is indicated on the drawing, and would also be expected as most FCC technologists design risers for dilute-phase conditions.
We have discovered that significantly improved operation is possible by separating and optimizing the desired operations which occur at the inlet to the catalyst regenerator. Refiners are trying to do two things, to mix hot regenerated catalyst with relatively cooler spent catalyst, and also to mix spent catalyst with air. The former ensures that the catalyst is supplied to the combustor at a uniform temperature with a uniform carbon distribution, and the latter ensures that there is a uniform supply of oxygen. These requirements must be met if uniform regeneration of the catalyst is to be achieved. Conditions which are optimum for good catalyst-catalyst mixing are not optimum for promoting good catalyst-air mixing.
Our inventive concept provides for a riser-mixer containing a relatively dense-phase, turbulent catalyst-catalyst mixing zone, a catalyst-regeneration gas mixing zone, situated above said riser and a combustor zone situated above the catalyst-regeneration gas mixing zone. The catalyst-regeneration gas mixing zone is often referred to herein as a transition section or transition zone, this being based upon the fact that the catalyst-regeneration gas mixing zone provides a transition from the lower riser-mixer to the upper combustor. We have discovered that by providing zones for each of the catalyst-catalyst and catalyst-regeneration gas mixing steps a significantly improved catalyst regeneration is made possible.
Dense-phase, turbulent conditions are maintained in the riser-mixer by severely limiting the amount of fluidizing gas which is added to the lowermost portion thereof. Air is preferably used as the fluidizing medium, because it is cheap and readily available and its presence permits some combustion to occur, though it is not essential to use air as the fluidizing medium. A relatively-small-diameter riser is necessary, the riser having a diameter typically one-fourth that of the combustor, to promote intimate mixing of regenerated and spent catalyst in the riser. A significant amount of combustion air is added at the transition section between the riser and the combustor to promote coke burning.
Intimate mixing of regenerated and spent catalyst occurs in the small diameter, dense bed riser-mixer. The spent catalyst is heated by hot regenerated catalyst, so that when spent catalyst contacts combustion air, coke burning readily occurs. In a preferred embodiment the transition zone between the riser-mixer and the combustor is in the shape of a frustum of a cone or of a funnel, wherein the bulk of combustion air is added to the transition zone through holes distributed about the surface of the funnel. When the angle between the center line of the riser and the surface of the funnel is about 45.degree., fabrication costs are minimized and a venturi acceleration effect is obtained which provides for excellent air and catalyst mixing. Good results can, however, be obtained with other angles.